The present application claims priority to Japanese Application No. P11-352349 filed Dec. 10, 1999, which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
This invention relates to a single-crystal silicon layer, its epitaxial growth method and semiconductor, which are suitable for use in thin-film transistors (TFT), for example.
2. Description of the Related Art
Heretofore, a typical method for epitaxially growing a single crystal silicon (Si) layer is to decompose and grow silane (SiH4), dichlorosilane (Si2Cl2H4), trichlorosilane (SiCl3H4), silicon tetrachloride (SiCl4), or the like, under the temperature of about 700 through 1200xc2x0 C., hydrogen atmosphere, pressure of 1.33xc3x97104 to 1xc3x97105 Pa (100 to 760 Torr) by using chemical vapor deposition (CVD).
However, the method of epitaxially growing a single crystal silicon layer by conventional CVD mentioned above involves the problem that the growth temperature is high. More specifically, in CVD, since energy required for chemical interaction and growth during epitaxial growth of a single crystal silicon layer is all supplied in form of heat energy obtained by heating a substrate to a high temperature, decreasing the growth temperature to or below 700xc2x0 C. will invite a crystallographic deterioration and a decrease of the growth rate. Therefore, it is not possible to decrease the growth temperature from about 700xc2x0 C. Further, when the growth temperature is in the range of about 1000 to 1200xc2x0 C., the ratio of decomposed reactant gas (silane, dichlorosilane, or the like) contributing to epitaxial growth of a single crystal silicon layer is around 1 to 5%. When the growth temperature is 800xc2x0 C., it further decreases to around 0.1 to 0.5%. Thus the efficiency of use of reactive gas is seriously low, and it invites an increase of its cost. Furthermore, since exhaust reactant gas (silane, dichlorosilane, and so forth) are harmful, a process for change it harmless, such as burning or absorption, is required to prevent the reactant gas from being released in the air. Therefore, when the efficiency of the use of the reactant gas seriously decreases as indicated above, the expense for the process for changing it harmless also increases.
It is therefore an object of the invention to provide a epitaxial growth method of a single crystal silicon layer capable of epitaxially growing a high-quality single crystal silicon layer at a temperature lower than that of conventional CVD; a single crystal silicon layer obtained by the method; and a semiconductor device using such a single crystal silicon layer.
The Inventor made researches toward solution of the above problems involved in conventional techniques. These researches are outlined below.
Recently, as a growth method of polycrystalline silicon layers and amorphous silicon layers, a growth method called catalytic CVD are being remarked (for example, Japanese Patent Laid-Open Publication No. hei 10-83988 and Applied Physics, Vol. 66, No. 10, p. 1094(1997)). This catalytic CVD uses catalytic cracking reaction between a heated catalyst and reactant gas (source material gas).
The Inventor made consideration about application of catalytic CVD to epitaxial growth of a single crystal silicon layer. That is, catalytic CVD, in its first stage, brings reactant gas (such as silane and hydrogen in case of using silane as the source material of silicon) into contact with a hot catalyst heated to 1600 through 1800xc2x0 C., for example, to activate the reactant gas and thereby make silicon atoms, or clusters of silicon atoms, and hydrogen atoms, or clusters of hydrogen atoms, having high energies, and in its second stage, raises the temperature of these silicon atoms and hydrogen atoms or molecules having high energies, or a substrate that supplies their clusters, to a high temperature, thereby to supply and support the energy required particularly for silicon atoms to align along the crystalline orientation of the substrate. Therefore, catalytic CVD enables epitaxial growth of a single crystal silicon layer even at a lower temperature than that of conventional CVD, such as around 350xc2x0 C., for example.
However, according to results of various experiments made by the Inventor, in the case where a single crystal silicon layer is epitaxially grown at a low temperature by existing catalytic CVD, oxygen is more liable to be brought into the growth layer than that epitaxially grown by conventional CVD, and the oxygen concentration in the single crystal silicon layer obtained often exceeds several atomic % (at %). This amounts at least to 5xc3x971020 atoms/cm3 (atoms/cc) when converted into atomic concentration. Since the maximum solution of oxygen in silicon is 2.5xc3x971018 atoms/cc (for example, Semiconductor Handbook 2nd Edition, pp.128-129, edited by Hisayoshi Yanai, Ohmusha, 1977), and the said oxygen concentration is far beyond the maximum solution of oxygen in silicon, 2.5xc3x971018 atoms/cc. When oxygen over the maximum solution is contained in silicon, oxygen precipitates forming silicon oxide, and sometimes results in forming an oxide thin film around silicon crystal grains or sometimes results in forming oxide grains with a further increase of oxygen.
Under the circumstances, the Inventor made researches about growth conditions for epitaxial growth of a single crystal silicon layer by catalytic CVD toward obtaining a high-quality single crystal silicon layer.
That is, repeated were experiments of epitaxially growing single crystal silicon layers by using catalytic CVD and variously changing process conditions under a low temperature range (200 through 600xc2x0 C.) and then evaluating them. As a result, it was found that, for growing high-quality single crystal silicon layers by catalytic CVD, conditions such as pressure of the vapor-phase growth atmosphere, partial pressure of oxygen and moisture in the growth atmosphere, and soon, were absolutely different from those of conventional CVD. More specifically, at least in the initial period of growth, the total pressure of the growth atmosphere was set to a much lower pressure than that of existing catalytic CVD, e.g., in the range from 1.33xc3x9710xe2x88x923 Pa to 4 Pa (from 0.01 mTorr to 30 mTorr), it was confirmed that the maximum oxygen concentration at least near the boundary with the substrate was as very low as 3xc3x971018 atoms/cc (0.006 at %), and high-quality single crystal silicon layers could be grown. Also when the partial pressure of oxygen and moisture in the growth atmosphere at least in the initial period of growth was set in the range from 6.65xc3x9710xe2x88x9210 Pa to 2xc3x9710xe2x88x926 (from 0.005xc3x9710xe2x88x926 mTorr to 15xc3x9710xe2x88x926 mTorr), it was confirmed that the oxygen concentration at least near the boundary with the substrate was similarly as very low as 3xc3x9710 atoms/cc (0.006 at %), and high-quality single crystal silicon layers could be grown. This partial pressure of oxygen and moisture can be obtained when oxygen and moisture around 0.5 ppm in total are contained in the reactant gas.
The present invention has been made through studies based on the above knowledge of the Inventor.
According to the first aspect of the invention, there is provided a single crystal silicon layer epitaxially grown by catalytic CVD on a material layer in lattice alignment with single crystal silicon, characterized in:
the maximum oxygen concentration thereof being 3xc3x971018 atoms/cm3 at least in a region having the thickness of 10 nm thick from the boundary between the material layer and the single crystal silicon layer.
In the first aspect of the invention, the maximum oxygen concentration at least in a region with the thickness of 10 nm from the boundary between the material layer in lattice alignment with single crystal silicon and the single crystal silicon layer is preferably not higher than 2xc3x971018 atoms/cm3. Further, the maximum oxygen concentration at least in a region with the thickness of 50 nm, or 100 nm, from the boundary between the material layer in lattice alignment with single crystal silicon and the single crystal silicon layer is preferably not higher than 2xc3x971018 atoms/cm3.
According to the second aspect of the invention, there is provided a single crystal silicon layer having a thickness not exceeding 1 xcexcm epitaxially grown by catalytic CVD on a material layer in lattice alignment with single crystal silicon, characterized in:
the maximum oxygen concentration thereof being 3xc3x971018 atoms/cm3.
In the second aspect of the invention, thickness of the single crystal silicon layer may be not larger than 100 nm, or not larger than 50 nm. Additionally, the maximum oxygen concentration is preferably not higher than 2xc3x971818 atoms/cm3.
According to the third aspect of the invention, there is provided a single crystal silicon layer epitaxially grown by catalytic CVD on a material layer in lattice alignment with single crystal silicon, characterized in:
being epitaxially grown by maintaining the total pressure of the growth atmosphere in the range from 1.33xc3x9710xe2x88x923 Pa to 4 Pa at least in an initial period of the epitaxial growth. According to the fourth aspect of the invention, there is provided a single crystal silicon layer epitaxially grown by catalytic CVD on a material layer in lattice alignment with single crystal silicon, characterized in:
being epitaxially grown by maintaining the partial pressure of oxygen and moisture in the growth atmosphere in the range from 6.65xc3x9710xe2x88x9210 Pa to 2xc3x9710xe2x88x926 Pa at least in an initial period of the epitaxial growth.
According to the fifth aspect of the invention, there is provided an epitaxial growth method for epitaxially growing a single crystal silicon layer by catalytic CVD on a material layer in lattice alignment with single crystal silicon, characterized in:
the total pressure of the growth atmosphere being maintained in the range from 1.33xc3x9710xe2x88x923 Pa to 4 Pa at least in an initial period of the epitaxial growth.
According to the sixth aspect of the invention, there is provided an epitaxial growth method for epitaxially growing a single crystal silicon layer by catalytic CVD on a material layer in lattice alignment with single crystal silicon, characterized in:
the partial pressure of oxygen and moisture in the growth atmosphere being maintained in the range from 6.65xc3x9710xe2x88x9210 Pa to 2xc3x9710xe2x88x926 Pa at least in an initial period of the epitaxial growth.
According to the seventh aspect of the invention, there is provided a semiconductor device having a single crystal silicon layer which is epitaxially grown by catalytic CVD on a material layer in lattice alignment with single crystal silicon, characterized in:
the single crystal silicon layer having the maximum oxygen concentration of 3xc3x971018 atoms/cm3 at least in a region thereof to be used as a carrier channel.
In the seventh aspect of the invention, the maximum oxygen concentration of the single crystal silicon layer is preferably not higher than 2xc3x971018 atoms/cm3.
The semiconductor device may be basically any that uses the single crystal silicon layer. Specifically, it may be a thin-film transistor (TFT), which is MISFET, or junction FET, bipolar transistor, or the like, for example. Thickness of the carrier channel region in TFT is typically around 10 through 100 nm. Furthermore, not limited to these transistors, the semiconductor device may be a diode, capacitor or resistor, as well.
In the present invention, growth temperature of the single crystal silicon film by catalytic CVD is, for example, 200 through 600xc2x0 C.
In the present invention, the base layer for epitaxially growing the single crystal silicon layer on, i.e. the material layer in lattice alignment with single crystal silicon, may be made of single crystal silicon, or sapphire, spinel, or the like. The xe2x80x9csingle crystal siliconxe2x80x9d is used to involve those including sub-boundaries.
According to the first aspect of the invention having the above-summarized structure, since the maximum oxygen concentration is not higher than 3xc3x971018 atoms/cm3, which is much lower than that of a single crystal silicon layer grown at a low temperature by existing catalytic CVD at least in a region with the thickness of 10 nm from the boundary between the material layer in lattice alignment with single crystal silicon and the single crystal silicon layer, a high-quality single crystal silicon layer can be obtained.
According to the second aspect of the invention having the above-summarized structure, since the maximum oxygen concentration is not higher than 5xc3x971018 atoms/cm3, which is much lower than that of a single crystal silicon layer grown at a low temperature by existing catalytic CVD, a high-quality single crystal silicon layer can be obtained.
According to the third and fifth aspects of the invention having the above-summarized structures, since the total pressure of the growth atmosphere is set in the range from 1.33xc3x9710xe2x88x923 Pa to 4 Pa at least in the initial period of epitaxial growth, partial pressure of oxygen and moisture in the growth atmosphere can be maintained in the range from 6.65xc3x9710xe2x88x9210 Pa to 2xc3x9710xe2x88x926 Pa at least in the initial period of epitaxial growth, and the amount of oxygen brought into the growth layer can be diminished significantly. As a result, the maximum oxygen concentration is not higher than 3xc3x971018 atoms/cm3, which is much lower than that of a single crystal silicon layer grown at a low temperature by existing catalytic CVD at least in a region with the thickness of 10 nm from the boundary between the material layer in lattice alignment with single crystal silicon and the single crystal silicon layer, and therefore, a high-quality single crystal silicon layer can be obtained.
According to the fourth and sixth aspects of the invention having the above-summarized structures, since the partial pressure of oxygen and moisture in the growth atmosphere is set in the range from 6.65xc3x9710xe2x88x9210 Pa to 2xc3x9710xe2x88x926 Pa at least in the initial period of epitaxial growth, the amount of oxygen brought into the growth layer can be diminished significantly. As a result, the maximum oxygen concentration is not higher than 3xc3x971018 atoms/cm3, which is much lower than that of a single crystal silicon layer grown at a low temperature by existing catalytic CVD at least in a region with the thickness of 10 nm from the boundary between the material layer in lattice alignment with single crystal silicon and the single crystal silicon layer, and therefore, a high-quality single crystal silicon layer can be obtained.
According to the seventh aspect of the invention having the above-summarized structure, since the maximum oxygen concentration is not higher than 3xc3x971018 atoms/cm3, which is much lower than that of a single crystal silicon layer grown at a low temperature by existing catalytic CVD at least in a region to be used as the carrier channel, the single crystal silicon layer has a high quality, and it is possible to obtain a high-performance semiconductor device like TFT having a high carrier mobility, by using this single crystal silicon layer.
The above, and other, objects, features and advantage of the present invention will become readily apparent from the following detailed description thereof which is to be read in connection with the accompanying drawings.